545411 Water As Key to Activity and Selectivity in Co Fischer-Tropsch Synthesis

Monday, June 3, 2019: 4:24 PM
Texas Ballroom EF (Grand Hyatt San Antonio)
Erling Rytter1, Øyvind Borg2, Nikolaos Tsakoumis3, Bjørn Christian Enger4 and Anders Holmen1, (1)Chemical Engineering, Norwegian University of Science and Technology, Trondheim, Norway, (2)Equinor, Trondheim, Norway, (3)Department of Chemical Engineering, NTNU, Trondheim, Norway, (4)SINTEF, Trondheim, Norway

Water as key to activity and selectivity in Co Fischer-Tropsch synthesis

Erling Rytter a,b, Øyvind Borg a,c, Nikolaos E. Tsakoumis a, Bjørn Christian Enger, b and Anders Holmen a

a Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim

b SINTEF Industry, N-7465 Trondheim, Norway

c Present address: Equinor Research Center, Trondheim, Norway <>1.    Introduction

Supported cobalt catalysts are widely studied and applied for conversion of synthesis gas in low-temperature (< 250 °C) Fischer-Tropsch (FT) synthesis. These catalysts provide high activity, high selectivity to long chain paraffins and low-water-gas shift activity. It is common practice to add a noble metal promoter, typically platinum or rhenium, to optimize the performance of the catalyst. Both cobalt and the promoter are rather expensive, and optimal usage in the reactor is of crucial importance. For all catalyst systems and process conditions, water added to the feed and indigenous water produced during synthesis result in higher selectivity to higher hydrocarbons. The C5+ selectivity increases with conversion due to a positive effect of produced water. Apparently, there is moderate effect of produced water on the activity. Nevertheless, in a recent paper it was suggested that water plays a key role in activation of CO by providing hydrogen to the oxygen atom.[1] Adsorbed water presumably interacts with CO and lowers the energy barrier for CO activation, thereby enhancing the surface coverage of polymerization intermediates. Added water to the syngas may increase or decrease the catalyst activity.[2] More inert supports and large pore size supports have a positive response.[3]

The present work concentrates on Re-promoted Co-catalysts on γ-alumina supports with large variations in pore sizes and pore size distributions,[4] as well as on α-alumina supports, and the effect of water. Differences in pore characteristics result in large variations in measured cobalt particle sizes. Effect of water is studied by adding water vapor during the FT-reaction. An attempt is made to unravel the relationship between pore structure and cobalt size with water response and selectivity. Most data reported are for lab. scale fixed-bed reactors. <>2.    Results

Thirteen γ-alumina supports with different pore sizes and pore size distributions, and three supports prepared by calcining γ-alumina at temperatures between 1130 and 1140 ⁰C to give α-alumina contents between 84 and 100%, have been impregnated with cobalt and rhenium. The catalysts were tested for Fischer-Tropsch synthesis (FTS) under dry and enhanced water vapor pressure conditions. It is shown that there is a positive correlation between pore size and cobalt crystallite size of the reduced catalyst after incipient wetness impregnation as long as the pore size distribution is sufficiently narrow. There is a concurrent increase in selectivity to C5+ with pore diameter due to higher chain propagation probabilities α3+. Higher α values are ascribed to larger cobalt crystallites that promote CO activation resulting in higher surface coverage of CHx monomers. Adding water vapor to the syngas feed has a strong positive effect on selectivity to higher hydrocarbons and activity. Water imposes a significant enhancement of α1 that is attributed to higher relative cobalt surface coverage of H2O(OH-) and CHx relative to hydrogen. Small pores are susceptible to condensation of water during FTS.

For the α-alumina supported catalysts, both activity and selectivity to higher hydrocarbons respond positively to water added or generated in situ by the reaction. A linear trend between formation of methane and C5+ products were found, but displaced to higher C5+ values compared to catalysts on γ-alumina; see Figure 1.

Figure 1. Methane selectivity plotted as function of C5+ selectivity for catalysts Cα1, Cα2, Cα3, Cγ (C11 and C9). See ref. 4 for notations.

Data points are for start and end of each test period. Five test periods were used: A: Synthesis gas with flow rate 250 ml/min. B: Synthesis gas with reduced space velocity to give an initial CO conversion of ca. 50% at 30 h time-on-stream (TOS). C: Keeping the synthesis gas flow-rate from period B and adding water vapor to give 21% water vapor pressure at the reactor inlet. D: Increasing the water vapor pressure to 35%. E: Returning to the conditions of period B.

Calculation of chain propagation probabilities (αn) for α-alumina supported catalysts confirms that the first step, characterized by α1, increases the most under higher water partial pressure. Moreover, α1 is significantly higher for α- compared to all γ-alumina supports irrespective of pore sizes of the latter; see Figure 2.

           

Figure 2. Chain propagation probabilities α1, α2, α3 and α4 in end of test periods A to E for two γ-alumina and one α-alumina supported catalysts. Catalyst notations in brackets refer to numbering in ref. 4; C10: average pore diameter 11.6 nm, C12: 18.3 nm.

These results, going from γ-alumina to α-alumina and increasing pore sizes, are ascribed to suppression of hydrogen coverage on the cobalt surface, linked to more regular cobalt crystallites, accompanied by enhanced water assisted generation of CHx polymerization monomers. Surprisingly, the following α2 probability is comparably low for α-alumina supports, although it increases significantly with water concentration. Linear correlations are found between each pair of parameters α1, α2, α4 and SC5+; giving support to a mechanistic model where all products are interlinked, including methane. Transients observed when water was added or removed from the system are ascribed to pore diffusion. Selectivities in these periods follow closely the general selectivity trends found for different process conditions. <>3.    Conclusion

4.     Cobalt particle size increases with pore size of the support material, specifically γ-alumina, when deposited through incipient wetness impregnation, drying and calcination.

5.     For broad or bimodal pore size distributions, cobalt preferably is deposited in the larger pores by the incipient wetness impregnation method.

6.     Site-time yield is essentially constant for cobalt crystallites in the range 8-13 nm for supports with large variations in pore size and pore size distribution.

7.     Broad pore size distribution in the catalyst causes a strong negative impact on catalyst activity when water vapor is added to the syngas feed.

8.     Narrow pores are susceptible to condensation of water although direct experimental evidence is lacking.

9.     The negative impact of water vapor on activity is strengthened for catalysts with narrow average pore size.

10.  Well adapted pores size distribution, wide pores and sufficiently large cobalt crystallites causes a positive response on activity during addition of water vapor in moderate concentrations.

11.  Water vapor pressure increases the selectivity to higher hydrocarbons with no apparent limit for the partial pressure.

12.  C5+ selectivity increases with pore diameter of the catalyst.

13.  There is a positive, but moderate, correlation between C5+ selectivity response to added water and pore diameter.

14.  A concurrent response for C5+ selectivity and activity on added water implies a common mechanistic origin suggested to be water activation of CO which impacts the concentration of polymerization monomers.

15.  The chain growth probabilities α3+ increase significantly as the pore diameter becomes wider. It follows that larger cobalt crystallites are more effective in CO activation.

16.  Water increases the α1 value significantly. This is attributed to suppressed hydrogen coverage on cobalt.

17.  Increase in C5+ with water partial pressure is mainly due to higher α1 and only to a minor extent to α3+.

18.  Deactivation by water reduces C5+ and increases α1 indicating higher hydrogen coverage.

Unique properties of catalysts on α-alumina are:

-           Very high α1 values at all process conditions

-           Low α2 that respond positively to water vapor concentration

-           Higher α3 and α4 probabilities that follow trend from γ-alumina supports.

 



<>4.    References

[1] E. Rytter and A. Holmen, Consorted vinylene mechanism for cobalt Fischer-Tropsch synthesis encompassing water or hydroxyl assisted CO-activation. Topics Catal., 61 (2018) 1024-1034.

[2] E. Rytter and A. Holmen, Perspectives on the Effect of Water in Cobalt Fischer–Tropsch Synthesis. ACS Catal. 7(8) (2017) 5321.

[3] E. Rytter and A. Holmen, On the Support in Cobalt Fischer-Tropsch-Emphasis on Alumina and Aluminates. Catal. Today 275 (2016) 11-19.

[4] E. Rytter, Ø. Borg, N. E. Tsakoumis and A. Holmen, Water as key to activity and selectivity in Co Fischer-Tropsch synthesis; γ-alumina based structure-performance relationships, J. Catal., 365 (2018) 334-343.


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